How is zakat calculated on two bank accounts, one of which changes its balance monthly, and is zakat due on a down payment for a car whose delivery was delayed for more than a year, and is zakat due on the sale price of the new car that was subsequently sold?

Any amount of money that has completed a full lunar year (hawl) is subject to zakat (definition), whether it is from salary or other sources. The amount you paid for the car is not subject to zakat. If you bought a car with the intention of using it, then sold it because you no longer needed it, its value is not subject to zakat until a full lunar year passes over it.